This PR gives the Navidrome web UI a 'proper' own service worker, not just the current boilerplate service-worker.js that gets auto-generated during the build process by CRA, which is outdated anyway since it loads Workbox v3 libraries instead of the current v6.1.2. Plus, that service worker caches stuff for offline usage, which breaks things.

Workbox, by the way, is pretty cool stuff: basically it's a set of Google libraries to make service workers easy. See: https://developers.google.com/web/tools/workbox

Changes in this PR:

• updated serviceWorker.js to load this new service worker
• named the new service worker navidrome-service-worker.js so it doesn't get overwritten by the CRA generated one
• added a new file offline.html containing an error message

So what does the service worker do?

• loads the Workbox libraries from a Google CDN (note: we don't have to load it from an external website, but this is also how Navidromes' current CRA-generated service worker does it)
• it installs itself, claims all clients (ie, Navidrome in other tabs) and immediately takes over from any previously running service worker (skipWaiting)
• it caches offline.html on install
• runs Navidrome with a "network-only" caching strategy (=fixes response served by service worker has redirections #904 )
• serves offline.html when the server is unreachable (=fixes service worker: "fetch" event handler needs to return a valid response when the device is offline #911 )

Please test this, for example:

• by changing between two different builds of the server and see if the fresh service-worker gets loaded
• log out, close tab, log in on various browsers / PWA platforms
• go offline, with both desktop and mobile browsers, check if offline.html gets displayed